1. Field of the Invention
The present invention relates to a throttle control apparatus which is attached to an internal combustion engine. More particularly, the present invention pertains to a throttle control apparatus which is arranged to control the degree of opening of the throttle valve of an internal combustion engine by means of a driving source, for example, a motor, in accordance with the operation of the accelerator, thereby effecting various control operations such as acceleration slip control, constant-speed running control, idling control, automatic vehicle speed limiting control, fuel-saving running control, etc., and which is designed so that the control effected by the driving source can be suspended if necessary.
2. Description of the Related Art
The throttle valve of an internal combustion engine is arranged to control the output of the internal combustion engine by controlling the fuel-air mixture in a carburetor or the intake air quantity in an electronically controlled fuel injector in association with an accelerator actuating mechanism which includes an accelerator pedal.
The accelerator actuating mechanism has heretofore been mechanically coupled to the throttle valve, whereas there has recently been proposed an apparatus wherein the throttle valve is opened and closed in accordance with the operation of the accelerator by a driving means which is interlocked with a driving source, for example, a motor. For example, Japanese Patent Laid-Open Publication (KOKAI) No. 55-145867 (1980) discloses an apparatus wherein a stepping motor is coupled to the throttle valve and driven in accordance with the operation of the accelerator pedal.
Japanese Patent Laid-Open Publication (KOKAI) No. 59-153945 (1984) lists examples of the countermeasures which have heretofore been taken when an electronically controlled actuator that drives the stepping motor falls into an uncontrollable situation in an apparatus of the type disclosed in Japanese Patent Laid-Open Publication (KOKAI) No. 55-145867 (1980). For example, the throttle shaft is disengaged from the electronically controlled actuator by means of an electromagnetic clutch and the throttle valve is returned to the closed position by means of a return spring. The same publication states that the prior art listed therein has no driving means for opening and closing the throttle valve after the control by the electronically controlled actuator has stopped and therefore is incapable of permitting the vehicle to be moved to a given place for repair, and proposes a solution to the problem.
Specifically, the proposed solution comprises an electromagnetic clutch interposed between a rotary shaft which rotates in response to the driver stepping on the accelerator so that, when excited, it disconnects the two shafts, whereas, when not excited, it connects them together. A control circuit drives a relay when detecting an abnormality of the control operation effected by the electronically controlled actuator to cut off the supply of power to the actuator and the electromagnetic clutch. Accordingly, when the electronically controlled actuator falls into an uncontrollable situation, the throttle shaft is mechanically coupled to the accelerator pedal through the electromagnetic clutch.
According to the technique disclosed in the above-described Japanese Patent Laid-Open Publication (KOKAI) No. 59-153945 (1984), the fact that the electronically controlled actuator is in an uncontrollable state is detected by another control circuit and the supply of power to the actuator and the electromagnetic clutch is suspended by this control circuit. After the control operation has been suspended, the throttle shaft and the rotary shaft that is mechanically coupled to the accelerator pedal are connected together through the electromagnetic clutch. The electromagnetic clutch is maintained in the state wherein it is coupled to the actuator even after the throttle shaft has been mechanically coupled to the accelerator pedal through the electromagnetic clutch. In the description of operation of one embodiment when the electronically controlled actuator suspends its control operation, no driving torque is generated from the motor and therefore the resistance to the accelerator stepping force is so small that there is no hindrance to the operation of opening and closing the throttle valve as effected by the driver stepping on the accelerator pedal.
However, the electromagnetic clutch that is used in such prior apparatus is structurally large and high in cost. Further, possibilities exist that not only the electronically controlled actuator will become uncontrollable but also the control circuit will become unable to operate. For example, there is a possibility that the throttle valve could be continuously driven to the open side by electrical pulses or the like. In such a case, even if a switching means is provided to suspend the supply of power to the electromagnetic clutch and couple the throttle shaft to the accelerator pedal, there is no means for activating the throttle valve to the closed side against the throttle shaft that is driven by the actuator and it is therefore difficult to ensure the desired throttle opening.
When the above-described situation occurs, the driver generally stops actuating the accelerator and applies the brakes. In the arrangement described above, however, the throttle valve continues being driven by the actuator unless the electromagnetic clutch is de-energized.